Combustion or fireflood methods are known for enhanced recovery of oil from viscous oil reservoirs.
Generally, the reservoir is locally heated and then oxygen is supplied to the oil bearing reservoir through one or more injection wells. The injection of oxygen sustains combustion of in-situ oil and forms a vertical combustion front which produces hot gases. The combustion front advances towards production wells spaced from the injection wells.
The known combustion processes may be generally characterized as comprising: a burnt zone closest to the injection well; a combustion front; a vapour zone; a condensation layer; an oil bank; and finally a cool region which oil must flow through to be produced from a well. The combustion progresses in essentially a plug flow manner. This plug flow progression experiences the following disadvantages: the lighter hydrocarbons are in a layer ahead of the combustion, leaving only variable quality coke behind as fuel; and it is difficult to supply and maintain adequate oxygen levels, for continued combustion, at the ever extending front.
Ideally, the combustion front remains vertical, extending throughout the depth of the reservoir. If the combustion front contacts the entire reservoir, then maximum production efficiency may be achieved.
Ultimately however, over time the hot gases rise and tend to move laterally through the upper reaches of the reservoir toward the production wells. This phenomenon is referred to as "overriding". The results of overriding are uneven areal distribution of the combustion front and premature breaking through of gases at one or more production wells. This latter situation is characterized by high gas flow rates coupled with high temperature and oxygen effects at the production well. The need to produce oil and water accompanied by a prolific combustion gas flow through a single production well leads to high entrainment of sand, the formation of emulsions, and poor oil recoveries. Further, the production well may be damaged by burning at the well. Excessive sand rates can plug screens and impair the operation of downhole production pumps.
It is therefore an objective of this invention to improve the production efficiencies of combustion front enhanced oil recovery techniques and reduce the risks to production equipment.